Automatic volume control system



Jan. 25, 193s,-

all

ACROSSLEY i-:r AL' AUTOMr-'IICl VOLME CONTROL SYSTEM Filed July 25,'1935 Qns:

f ,du-"PED Cpo SL EEBBEQETE E/NEWA,

. ATTORNEY.

30 modulator substantially constant in'spite of wide requirements areobjectionable in any receiver; 30

45 eral different sets of conditions; and

' 5o Vacuum-tube amplifiers of the type herein con- The simplest formsof automatic amplication 50 Patented Jan. 25V, 1938 I I l I UNITEDSTATES lPATENTA ol-Fica 2.106.207 i AU'roMA'rIc vomlun CONTROL SYSTEMAlfred .Crossley and Herbert E. Meinema, Chicago,

Illa assignors to Johnson Laboratories, Inc., Chicago, Ill., alcorporation of Illinois a Application July z5, 1935, serlal No. 33,113

8 claims. (cl. 25o- 20) This invention relates to improvements invachigh-frequency signals for the transmission of uum-tube amplifyingsystems. More specifically, intelligence.v Although the-arrangements ofthe this invention relates to improvements ,in the present invention areapplicable to vacuum-tube means for automatically regulating theampliamplifiers-of all types, they are Iof especial advancation of avacuum-tube amplifying system. 'tage in the high-frequency amplifiers ofradio re- 5. An object of thisinvention is to provide animceivers. Inthis case, it is usually desirable that proved method of amplificationcontrol which au-v the output signal level remain substantiallycontomatically maintains the output level of a vacstant in spite of widevariations in the input signalv uum-tube amplifier substantially.constant in voltage due to fading or to the. Widely different 10 spiteof wide variations in the input voltage. signal strengths whichareencountered in tuning 10 Another object of the present invention is tofrom one signal to another from a diierent source. provide a system ofautomatic amplification con- The usual radio receiver consists of ahigh-fretrol which is responsive to relatively small changesquencyamplifying system, a demodulator, and a in the voltage at tireoutput of` a vacuum-tube low-frequency amplifier. Since the demodulator15 amplier. performs most satisfactorily within a relatively A furtherobject of this invention is to provide narrow range of signal levels, itis the usual pracmeans whereby the control of amplification which ticetoregulate the amplification of the highis automatically applied to avacuum-tube amfrequency amplifying system either manually or pliileris'self-increasing. automatically, in such a'way as to maintain theStill another object of the present invention is signal at thedemodulator substantially constant to provide an amplified controlvoltage for the despite large variations in the strength of theinautomatic regulation of the amplification of a coming signal. Ifthisoperation is performed yvacuum-tubo amplifier, without the use ofaddimanually, constant attention on the part of the tional vacuum tubesother than a demodulator. user is vrequired to compensate for therelatively An additional object of this invention is to rapid changes inthe incoming signal strengtln 25 provide an automatic amplificationcontrol system Furthermore, whenever the receiver is adjusted to whichmay be satisfactorily employed in conjuncreceive a different station themanual amplication with the high-frequency amplifier of a radio tioncontrol must be re-adjusted to produce the receiver, to maintain thesignal level at the dedesired output level from the new signal. Thesevariations in the .input signal voltage. and are especiallydisadvantageous in the case of Still a further object of the presentinvention is 'receivers intended for mobile use, as in automoto providean automatic amplification control sysbiles or motor boats. tem whichhas a desirable input vs. output char- Various arrangements have beenemployed in acteristic, but which does not prevent the full the past toAautomatically vary the amplification utilization of the amplificationof a vacuum-tube of the high-frequency amplifying system of a.amplifying system. radio receiver in accordance with changes in theAlthough a discussion of the principles underinput signal level. Thesearrangements are lying our invention is necessary to a full underusuallyoperated by rectifying a portion of thestanding thereof, certainportions of this specificaoutput voltage, and applying the rectifiedvolt-T40 tion will be bestunderstood when taken in con- 'age to acontrol electrode of one ormore of the ynection with the drawing, inwhich high-frequency amplifying vacuum tubes, there Fig. 1 is a graphshowing the usual automatic by regulating the amplification of thesystem in amplification control characteristics under sevapproximateaccordance with the amplitude of n the input signal. In somel cases, thecontrol is Fig. 2 is a schematic wiring diagram of a porapplied also ta-low-frequency amplifying vaction of a radio receiving apparatusincorporating uum tube, the aim being to supplement the reguourinvention and serving as an illustrative emlation of amplification whichis obtained in the bodiment thereof. l high-frequency portion of thereceiver.

templated are used in both low-frequency and control systems decreasethe amplification as soon high-frequency communication systems. Lowasany output signal is present, the degree of frequency amplifiers arecommonly employed in reduction increasing as the output signal increasesl telephone systems. High-frequency ampliers -in amplitude. Thesearrangements prevent the are widely used in systems employing modulatedfull utilization of the high-frequency ampliflca- 55 level.

tion of the receiver, since the amplification is reduced by any signalat the output. `Furthermore, the reduction in amplification does notkeep pace with the increasing input signal voltage, with the result thatthe output level inbe introduced by suitable means. In 'either case,

delaying the action of the 'automatic control permits the fullsensitivity of the receiver to be utilized.

The problem of decreasing the amplification at a sufficiently high rateto keep pace with an increasing input signal is especially acute in thecase of small receivers having only a limited number of high-frequencyamplifying vacuum tubes to which the control voltage may be applied. Thenumber of stages which, may be controlled is further limited by the factthat a vacuum tube which drives a' diode rectifier should .preferably beoperated at a fixed bias voltage.

With only a limited number of vacuum tubes to be controlled, therefore,the only alternative is to amplify tlie output voltage from therectifier before applying it to the control electrodes of one or morehigh-frequency amplifying vacuum tubes.

Amplification of the control voltage has, prior to the presentinvention, required either an additional vacuum tube or a substantialsacrifice in the effective plate potential available for the controlledamplifying Y vacuum tubes, or both. The arrangements of the presentinvention, however, provide for the amplification of the control voltagewithout the necessity for an additional vacuum tube and without anappreciable sacrifice in effective plate potential on the controlledamplifying vacuum tubes. The advantage of such an arrangement is obviousin any receiver, and this is especially the case in receivers intendedfo'r mobile service, where extreme compactness, wide-range automaticamplification control and economical operation are essential features ofa satisfactory receiver. Furthermore, the arrangement of the presentinvention includes provision for obtaining a desirable delayed operationof the automatic amplification control, permitting the full sensitivityof the receiver to be efficiently utilized.

'I'he performance characteristics of various forms of automaticamplification control systems are shown graphically in Fig. 1. Curve Arepresents the ideal form, the output rising rapidly to the normal leveland thence remaining constant atthis level. This characteristic is notattained in any simple arrangement. Curve B shows the performance of asimple control system in which a portion of the output signal isrectified and used to control the amplification of one or more precedingamplifying tubes. is required to produce normal outputand normal outputis reached very graduallyas the inputincreases. Curve C represents theperformance of a system similar to that of curve B, except that a delayeffect has been introduced to prevent the automatic controi-v from4operating i until the. output almost reaches its normal level. Fromthis.

A very large input-epoint on, however, the output increases appreciablyas the input increases. Curve D is the charaeteristic of a systemaccording to the presentl invention, 'in which provision Vis made forboth delaying and amplifying the control voltage which is applied to theamplifying vacuum tubes. In this case, the outputv soon reaches itsnormal level, and is closely maintained at this level as the inputincreases many times. closely approaches the ideal curve, indicatingthat the performance of the systemof the present invention is verysatisfactory.

Although the arrangements of the present invention may be applied tovarious types of amplifying systems, the embodiment shown in Fig. 2 asan illustrative example consists of a threestage high-frequencyamplifier followed by a diode rectifier. 'I'his type of amplifier iscommonly employed in radio receivers. The diode rectifier may be usedsimultaneously as the control rectifier and as the signal detector ordemodulator, or a separate rectifier may be employed as the demodulator.Both forms are within the scope of the present invention.

Referring to Fig. 2, high-frequency amplifying vacuum tubes I, 2 and 3,respectively, are cou- -pled in cascade by means of transformers 4, 5

and 6. Each transformer secondary is tuned to `the signal frequency byvariation of a capacitor 1, or by variation of the effective inductanceof the secondary itself. By-pass capacitors 8 furnish a low-impedancepath for all high-frequency currents. Amplifying vacuum tube 3 has inits plate. circuit, inductor 9 and resistor I0, Aand its plate 30 iscoupled to the anode II of rectifier vacuum tube I2 by blockingcapacitor I3. Diode load resistor I4 is connected between anode i II andthe junction of resistors 23 and 24. The

cathode 21 of vacuum,tube 3 is connected to ground through resistors 23and 24. The directcurrent grid'return of vacuum tube 3 is connected tothe junction of resistors vII and I8. 'I'he input signal to theamplifier is applied to Vprimary 26 of coupling transformer 4.

Vacuum tube I2 is a duplex diode, and includes a second diode anode 33which in connection with cathode I5 is employed as the demodulator, theaudio-frequency voltage being developed across resistor 34, connectedbetweendiode anode 33 and cathode I5. The amplified signal is fed todiode anode 33, through blocking capacitor 32.

When no input signal is present, an initial bias voltage for vacuumtubes I, 2 and 3 is developed across resistor 23. The values ofresistors 2| and 22 are so chosen that the cathode 28 of vac-* uum tube2 has the same potential relative to ground as the cathode 25 of vacuumtube I. In the absence of a signal, there is no appreciable voltageacross resistor i4.

When a signal is applied to. primary 2S and amplified by vacuum tubes I,2 and 3, and it exceeds the negative delay voltage on the diode anode IIdue to the voltage drop across resistor This curve 23, it will berectified by vacuum l voltage appears across resistor Il.

tube n. as.

soon as rectification takes place, a direct-current This voltage adds tothe initial bias on vacuum tubes I, 2-and l, thereby tending to reducetheir mutual con-' ductance and hence their amplication.- A dey vf'creasein voltage drop across'resistor 23 occurs when the platel currentsof vacuum tubes I and I. 3 decrease due to the increasing negative gridylo uum tubes.

I' ase bias which is being applied to these vac- 'Ihis change in thevoltage' drop across resistor 23 decreases the initial bias voltwhich isapplied to'the grids yof vacuum tubes .I, 2 and V3, but simultaneouslythe delay voltage on diode anode II is decreased and permits thedevelopment of a larger control voltage y amplification of the system.

' however, the left-hand terminal of resistor becomes less positive withrespect to ground.

Since the cathode 28 of vacuum tube 2 is at a substantially fixedpositive across resistor Il. The nt result, if this were the only .meansfor automaticallyy regulating the would be a perfomance characteristicsubstantially like that represented by curve C of Fig. l.

As the voltage across resistor 2l decreases, I4

potential relative to Y ground, the effective negative' bias on grid 3|of vacuum vtube 2'increases. This increase is in addition 'to the biasvoltage developed across diode load resistor Il. These two arrangements.combine to produce a very desirable self-increase in the overall'degree of amplification control, so'that the performance of the systemis similar to that represented by curve D of Fig. l.

, derstood by noting that, in effect, vacuum tube I "featureisespecially y Y mobileservice, where great variation in the fieldl Theoperation of the system willbe better unacts as a direct-currentampliiier. Resistors 23 and 24 are in the direct-current plate circuitof vacuum tubes I and 3, and any change in the direct-current voltageongrlds 29- changes they.

strengths of the signals is commonly encountered. l

It will be understood that vacuum tube I2 may l' be replaced by anyother suitable type of recti- `er, asfor example a copper-oxiderectifier. In alpreferred embodiment, inductor 9 is resonantv at afrequency approximately midway between the extreme vfrequencies to whichthe system may `be tuned, and its resonance characteristic isunderstood,l however,

without departing from 'increased control voltage being applied to onebroadened by means of resistor III. It will be meansymay be employed forcoupling amplifying vacuum tube 3 to rectifier I2. The arrangements ofthe present invention may be applied to amplifiers having more or lessthan three stages the scope thereof, the

or more amplifying vacuum tubes, some or all of which may be .theintermediate-frequency amplifiers in a superheterodyne type of receivingapparatus; and the increased control voltage may also be applied totubes which are subsequent to the demodulator, within the scope of thisinven- I tion.

.be understood. however,

. of said first vacuum vacuum tube acts additionally as a'direct-currentthat any other suitable" Athe cathode of said third tube when noaccording -to` Fig. .2, the following components and constants will befound satisfactory. It will that any lof these values may be altered,and theyare not to be taken as in any way' limiting the scope of thepresent invention.

Reference numeral Type or value .Type78 Typ@ 78 TypeB? (pentode portion)Type 6B7 (diode portion) 0.1 microiarad 0. 4 miilihenry 5000 ohms 200micromcrofarnds i. 0 megohm 0. l-megohm l. 0 megohm 0. 0l microiarud 750ohms 25,000 ohms 750 ohms 750 ohms used in the arrangements shown inFig. 2 being I used for any other desired purpose, as for example as afirst stage of low-frequency ampliflcation.

Having thus described our invention, what we claim is:

1. In a ra'dio receive a multi-stage amplifier including pluralamplifying vacuum tubes, a first means for automatic control of theamplification of a first and ajsecond of said vacuum tubes, and a second`means for controlling the amplification of said secondvacuum tube, saidsecond means including plural resistors inthe cathode circuit tubewhereby 'said first amplifier to increase the degree oiv control exertedupon said second vacuum tube by said first -control means.

f 42. In a multi-stag vacuum-tube amplifier, means for automaticamplification control including a diode rectifier coupled to the outputof said amplierand having/a direct-current connection from its negativeoutput terminal to a' control electrode of at least a first and a'secondvacuum tube of said amplifier, means for successively delaying andincreasing the effect of said automatic amplication control. includingAa, connection between the .cathode of said first vacuum tube and thecathode of a third vacuum tub'e; a connection between the cathode ofVsaid diode rectifier and the cathode of said third vacuum tube; aconnection includinga first and a second resistor vbetween vacuum tubeand ground; Aa connection between the junction of said first andsecond`resistors`and the positive terminal of a third resistor in theoutput circuit of said diode rectifier; and means for maintaining thecathode of said second vacuum tube at substantially the same potentialrelative to ground as that of the cathode' of said rst vacuum inputvoltage is applied to said amplifier..

coy

high-frequency vacuum-tube amplifier and a diode detector-rectifier,means for automatically regulating the amplification of said amplifierin accordance with changes in the direct-current output of said diodedetector-rectier, said means including a direct-current connectionbetween the anode of said diode detectorrectifier andA a controlelectrode of at least a first and asecond vacuum tube of said amplifier;a connection between the cathode of said first vacuum tube and thecathode of a third vacuum tube; a connection includinga first and asecond resistor between the cathode of said third vacuum tube andground; a connection between the junction of said first and secondresistors and the positive terminal of a third resistor in the outputcircuit of said diode detector-rectifier; a connection between thecathode of said third vacuum tube and the cathode of said diodedetector-rectifier; and means for maintaining the cathode of said secondvacuum tube at substantially the same potential relative to ground asthat of the cathode of said first'vacuum tube when no input is appliedto said radio receiver.

4. In a -radio receiver, a multi-stage high-frequency vacuum-tubeamplifier. the last vacuum vtube of which has its cathode in common withthe cathode of a diode rectifier coupled to the output of saidamplifier, and means for automatically regulating the amplification ofsaid amplifier in accordance with changes in the directcurrent output ofsaid diode rectifier, said means including a direct-current connectionbetween the anode of said diode rectifier and a control electrode of atleast a first and a second vacuum tube of said amplifier;a .connectionbetween the cathode of said first vacuum tube and' the cathode of saidlast vacuum tube; a connection including a first and a second resistorbetween the cathode of said last vacuum tube and ground; a connectionbetween the junction of said first and second resistors and the positiveterminal of a third resistor in the output circuit of said dioderectifier; and means for maintaining the cathode of said second vacuumtube at substantially the same potential relative to ground as that ofthe cathode of said first vacuum tube when no input voltage is appliedto said radio receiver.

5. In a radio receiver, means for successively delaying and increasing.the rang/,e of the amplification control `of a diode rectifier coupledto the output of a multi-stage high-frequency vacuumtube amplifier andhaving a direct-current connection from its anode to a control electrodeof at least a first and a second vacuum tube of said amplifier, saidmeans'including a. connection between the cathode of said first vacuumtube and the cathode of a third vacuum tube; a connection including` afirst and a second resistor between the cathode of said third vacuumtube and ground; a, connection between the junction of said first andsecond resistors and the positive terminal of a third resistor in theoutput circuit of said diode rectifier; a connection between the cathodeof said 4third vacuum tube and the cathode of said diode rectifier; andmeans for maintaining the cathode of' said second vacuum tube atsubstantially the same potential relative to ground as that of thecathode of said first vacuum tube when no input voltage is applied tosaid radio receiver.

6. In a radioreceiver, plural high-frequency amplifying tubes and adetector, means including a diode rectifier for automatic control of theamplification of at least some of said vacuum tubes, and means wherebyone of said controlled tubes acts additionally as a direct-currentamplifier to increase the degree of control exerted upon at least oneother of said vacuum tubes by said automatic control means.

7. In a radio receiver, plural high-frequency amplifying 4tubes and adetector-rectifier, means for automatic control of the amplification ofat vleast some of said vacuum tubes, and means whereby one of saidcontrolled tubes acts additionally as a direct-current amplifier toincrease the degree of control exerted upon at least one .other of saidvacuum tubes by said automatic control means.

8.'In a multi-stage amplifier, plural amplifying vacuum tubes, means forautomatic control of the amplification of at least some of said vacuumtubes including a. rectifier coupled to the output of said amplifier,and means whereby one of said controlled tubes, in the presence of aninput signal to said amplifier exceeding a pre-determined minimum value,acts additionally as a direct-current amplifier to increase the degreeof control exerted upon at least one other of said vacuum tubes by saidautomatic control means.

ALFRED cRossLEY. HERBERT E. MEINEMA.

